How to Design a PCB Layout

Introduction

Printed circuit board (PCB) layout design is a complex engineering art involving the layout of components and interconnections on a PCB to realize the circuit schematic functionality. A good PCB layout ensures proper signal and power integrity, electromagnetic compatibility, thermal management, manufacturability, and reliability of the product. This article provides a step-by-step guide on designing effective PCB layouts.

PCB Layout Design Steps

The major steps involved in designing the layout for a PCB are:

1. Planning the layout and creating a stackup
2. Placing components strategically
3. Routing traces taking signal integrity into account
4. Adding power/ground planes and ensuring decoupling
5. Incorporating thermal management features
6. Adding mounting holes, connectors, indicators, etc.
7. Finalizing layer stacks and interfaces
8. Checking design rule and manufacturing compliance
9. Validating with DFx analysis like signal, power, thermal, EMI
10. Iterating to optimize based on analysis feedback

Proper planning is key before starting the actual layout to avoid sub-optimal results requiring rework.

1. Layout Planning and Stackup Design

The first step is planning the layout architecture and defining the PCB layer stackup.

Key planning activities:

• Understand PCBspecs – board dimensions, layer count, density, etc.
• Review schematic for component types and counts
• Plan partitioning for analog and digital sections
• Define interfaces, high speed routing needs
• Plan power architecture and decoupling strategy
• Identify high power components needing cooling
• Understand enclosure and assembly constraints
• List critical nets needing impedance control
• Gather applicable routing guidelines from IPC and OEMs

Defining layer stackup:

• Select number of layers suitable for density
• Choose dielectric materials based on performance
• Determine copper weights for current needs
• Add impedance control layers if needed
• Assign plane layers (power, ground)
• Plan signal routing layers
• Consider double-sided component placement
• Incorporate internal thermal vias/layers if necessary
• Specify thickness, finish and solder mask for outer layers

Careful planning and stackup design ensures effective layout of all sub-systems.

2. Component Placement

Next step is intelligently placing components on the board.

Placement guidelines:

• Group associated circuits together
• Ensure important nets have short paths
• High speed ICs close to connectors
• Match component footprint to placement side
• Distribute heat sources avoiding hotspots
• Allow access to testpoints
• Ensure components fit within board outline
• Maintain clearance between components
• Standardize orientation for polarized parts
• Consider rework access requirements
• Define placement zones for partitioned layout

Good component placement minimizes interconnect lengths, noise coupling, and thermal issues while taking assembly needs into account.

3. Signal Trace Routing

With components placed, signal interconnects between pins are routed:

Signal trace routing tips:

• Use appropriate trace widths based on current
• Minimize length for critical signals like clocks
• Avoid 90° angles. Use 45° bends.
• Route noise-sensitive signals away from aggressors
• Provide isolation channels between digital and analog
• Use impedance matching techniques if needed
• Take care of high speed interfaces
• Facilitate test probe accessibility
• Enable visual inspection where needed
• Allow space between traces for manufacturing

Intelligent trace routing controls impedance, EMI and signal quality while enabling testability.

4. Power Distribution and Decoupling

Proper PCB power distribution is key for stable functioning of circuits.

Power distribution considerations:

• Use power/ground planes to distribute current
• Decide on split or contiguous planes
• Stack-up should sandwich signal layers between power layers
• Use wide traces/polygons for power connections
• Add local vias in pads to connect devices to power plane
• Include thick interconnects between layers

Decoupling guidelines:

• Place bypass caps close to ICs on same layer
• Minimize trace length between cap and pin
• Select suitable capacitors for HF and LF decoupling
• Add sufficient bulk capacitance distributed around the board

Together, a robust power distribution network and decoupling strategy provide clean stable supply voltages to all devices.

5. Thermal Management

Proper cooling provisions must be incorporated for heat generating components:

Thermal design techniques:

• Identify components needing heatsinks from power dissipation
• Position hot parts for maximum heat sink contact
• Ensure air flow access over heat sinks and vents
• Use thermal vias under hot device pads
• Add internal thermal layers connected by vias
• Incorporate thick copper planes for spreading heat
• Define thermal pads for device cooling
• Check for hot spots and temperature gradients

This removes heat efficiently from critical high power devices.

6. Mechanical Features

Additional mechanical elements are added:

• Mounting holes with correct diameter and annular ring spacing
• Edge connectors, testpoints, indicators and switches
• Brackets, clamps and strengtheners if needed
• Mark component IDs, polarity, ratings as needed
• Add board outline with proper corner chamfers
• Include any required assembly instructions

These features facilitate mounting, assembly and usage of the designed PCB.

7. Finalizing Layer Stack

With routing completed, the individual layers are finalized:

• Review all routing on layers, rearrange if needed
• Check for manufacturing spacing violations
• Verify alignment between layers for vias
• Add reference markers for layer alignments
• Insert testpoints for probing individual layers
• Check plane void areas affecting current flow
• Define minimum annular rings for vias
• Confirm margins from edge meet requirements

This completes the detailed inner layer builds ready for integration.

8. Design Rule Checks

The PCB layout is then validated against:

• Electrical rules: spacing between traces, pads, and planes based on voltage levels and insulation needs
• Routing rules: trace widths and clearances, via dimensions, acute angle avoidance
• Manufacturing rules: capabilities of PCB fabrication process like minimum track width, hole size, spacing

Tools like designersRule inside Cadence Allegro can automate checking against IPC and OEM guidelines. Errors must be fixed to ensure manufacturability.

9. DFx Analysis

The next step is verifying the design using DFx simulations:

• Signal integrity: Check for reflections, crosstalk, timing issues using IBIS models
• Power integrity: Simulate power distribution network stability and resonance
• Thermal: Verify temperature profiles using tools like IcePak
• EMI/EMC: Model radiated and conducted emissions
• Mechanical: Stress analysis, vibration and shock checks

This validates the design meets all functional requirements before release.

10. Layout Optimization

Based on the analysis feedback, layout issues are addressed:

• Tune trace widths, spacing, layer stackup issues
• Adjust placement to minimize coupling
• Add shielding, bandgaps, power islands if needed
• Improve heat spreading and airflow
• Tweak decoupling strategy based on resonance modes
• Adjust trace angles, impedance matching
• Modify plane shapes to lower resonant peaks
• Reroute signals affecting EMC/EMI

With iterations, an optimized layout satisfying electrical, thermal, and mechanical needs is finalized.

Conclusion

• PCB layout design requires carefully planning the partitioning, layer stackup, placement strategy and routing architecture.
• Components must be intelligently placed to minimize interconnect lengths and noises.
• Signal traces should use controlled impedance routing to ensure signal integrity.
• A robust power distribution network and decoupling strategy stabilizes power delivery.
• Thermal design techniques like thermal vias, pads and internal layers enable cooling.
• Mechanical features are added to facilitate assembly, usage and testing.
• Extensive design validation using DFx analysis uncovers issues requiring tuning.

Using these best practices helps create a manufacturable layout optimized for electrical, thermal and mechanical performance. This results in a reliable PCB with the best signal and power integrity for the desired application.

Frequently Asked Questions

What are some key aspects to check during layout review?

Critical items to check in layout review are: impedance matching on high speed nets, bypass cap placement, plane void areas, clearance between traces and pads, trace angles, thermal reliefs on pads, vias aligned with pads, plane splits, and manufacturability spacing checks.

What is the optimal copper thickness for power traces?

For power traces carrying over 1A current, it is recommended to use thicker 2oz/3oz copper instead of standard 1oz. This significantly reduces voltage drop over interconnects due to lower resistive losses.

How can EMI emissions be reduced through PCB layout?

EMI reduction techniques include: enclose board in grounded metal shield, use multilayer board with uninterrupted ground planes, route high speed traces over plane, use ground vias for shields, avoid big current loop areas, avoid slits/voids in planes, filter connectors.

What are some thermal vias best practices?

Use thermal vias under high power component pads. Each via should be 10-20 mils diameter with 1 oz copper plating. Include 4-8 vias in pad with 50% copper fill. Use thermal spokes or patterns connecting to internal ground layers which act as heat sinks.

What are some key signal integrity checks during PCB layout?

Critical SI checks include: match net trace impedance, minimize discontinuities, avoid stubs, route clock nets with daisy chains, use differential pairs with skew control, provide shielding for noise-prone signals, avoid 90° angles, use plane cavities below, add termination resistors.